The 8 and GTX2xx series don't play nice together. I'd wait for LCB001 to see this thread (or even send him a PM), he knows all about this.

3.0charlie is correct, unless the new drivers have fixed this mixing 2xx series cards with anything older dosn't work you get nothing but EUE's. And unless you want to deal with the Shader Bug, the only card you can mix with a 260 is another 260 with the same # of shaders.

BTW the new 250 does not count as it's just a rebadged 9800GTX+, not based on the new 200 series architecture...

Funny enough I've had more success getting a GTX 2xx card folding in the same box with a 4850/4870 or 4890 than with a G8x or G9x nVidia board. You should be able to use the 8800/9800GT for PhysX but not folding in the same box.

As for shaders, the only rule to follow is to have two cards with the same shader/stream count, or one that divides directly into another like a 32 or 64 shader with a 128 shader card, otherwise performance suffers for the card with fewer shaders.